Practice: Magnetism and Induction Introduction
Solutions
Level 1
1.
The rate of change of flux has which of the following units
A) farads B) joules C) volts D) m/s E) webers
2. A square loop is placed in a uniform magnetic field perpendicular to the plane of the loop as shown. The loop is
0.50 meters on a side and the magnetic field B has a strength of 2 T. If the loop is rotated through an angle of
90° in 0.1 second what would be the average induced EMF in the loop?
A) 0.025 C B) 0.40 V C) 5 V D) 10 V E) 80 V
Use ε = ΔФ / t
ε = ( BAf – BAi ) / t
ε = (0–(2)(0.5x0.5))/0.1
3.
Level 2
4.
Two light wires are hung vertically. With electrical current in both wires directed upwards
A) the wires will experience a force of attraction
B) the wires will experience a force of repulsion
C) the force on the right hand wire will cancel the force on the left hand wire
D) both wires will experience a torque until they are at right angles to each other
E) none of the above
Parallel current wires with same direction current attract.
5. The conventional current I in a long straight wire flows in the upward direction
as shown in the figure. (Electron flow is downward.) At the instant a proton of
charge +e is a distance R from the wire and heading directly toward it, the
force on the proton is:
Answer: E
6. A uniform magnetic field B that is perpendicular to the plane of the page now passes through the loops, as
shown. The field is confined to a region of radius a, where a < b, and is changing at a constant rate. The induced
emf in the wire loop of radius b is ε. What is the induced emf in the wire loop of radius 2b ?
(A) Zero (B) ε/2 (C) ε (D) 2ε (E) 4ε
Since both loops contain the same value of BA and it is changing the same for both of them, the
quantity ΔBA/t is the same for both so both have the same induced emf.
The figure shows a bar moving to the right on two conducting rails.
7. To make an induced current i in the direction indicated, in what direction would the magnetic field be in the
area contained within the conducting rails?
A) out of the page D) to the left
B) into the page E) It is impossible
C) to the right
The rail makes a loop of wire as shown by the current flow. Using Lenz law, as the loop expands
with the motion of the bar, it is gaining flux lines in whatever direction the B field is and the
loop current flows in a direction to oppose that gain. Using RHR–solenoid for the single
loop, the B field induced is directed out of the page so it must be opposing the gain of B field
that is already there going into the page.
8. When a wire moving through a magnetic field has a voltage induced between the wire’s ends, that voltage is
I. directly proportional to the strength of the magnetic field
II. directly proportional to the velocity of the wire
III. directly proportional to the diameter of the wire
A) I only B) II only C) III only D) I and II only E) II and III only
Based on ε =BLv
Jedi
9. A wire of constant length is moving in a constant magnetic field, as shown.
The wire and the velocity vector are perpendicular to each other and are
both perpendicular to the field. Which of the following graphs best
represents the potential difference E between the ends of the wire as a
function of velocity?
Answer: A
Based on ε =BLv, its a linear variation
Free Response
Solution